Horton

Re: On login trigger: take three

Alexander Korotkov <aekorotkov@gmail.com>

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From: Alexander Korotkov <aekorotkov@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Daniel Gustafsson <daniel@yesql.se>, Mikhail Gribkov <youzhick@gmail.com>, Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>, Greg Nancarrow <gregn4422@gmail.com>, Ivan Panchenko <wao@mail.ru>, Teodor Sigaev <teodor@sigaev.ru>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, vignesh C <vignesh21@gmail.com>, Pavel Stehule <pavel.stehule@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>
Date: 2023-10-13T08:26:45Z
Lists: pgsql-hackers 
On Fri, Oct 13, 2023 at 4:18 AM Robert Haas <robertmhaas@gmail.com> wrote:
> On Thu, Oct 12, 2023 at 6:54 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> > On Thu, Oct 12, 2023 at 8:35 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> > > Doesn't that mean that if you create the first login trigger in a
> > > database and leave the transaction open, nobody can connect to that
> > > database until the transaction ends?
> >
> > It doesn't mean that, because when trying to reset the flag v44 does
> > conditional lock.  So, if another transaction is holding the log we
> > will just skip resetting the flag.  So, the flag will be cleared on
> > the first connection after that transaction ends.
>
> But in the scenario I am describing the flag is being set, not reset.

Sorry, it seems I just missed some logical steps.  Imagine, that
transaction A created the first login trigger and hangs open.  Then
the new connection B sees no visible triggers yet, but dathasloginevt
flag is set.  Therefore, connection B tries conditional lock but just
gives up because the lock is held by transaction A.

Also, note that the lock has been just some lock with a custom tag.
It doesn't effectively block the database.  You may think about it as
of custom advisory lock.

------
Regards,
Alexander Korotkov

Commits

  1. Fix some typos in event trigger docs

  2. Use heap_inplace_update() to unset pg_database.dathasloginevt

  3. Remove the flaky check in event_trigger_login regression test

  4. Fix instable 006_login_trigger.pl test

  5. Add support event triggers on authenticated login

  6. Add GUC for temporarily disabling event triggers

  7. Fix typo in reference to __FreeBSD__.

  8. Restore robustness of TAP tests that wait for postmaster restart.

  9. Restore the portal-level snapshot after procedure COMMIT/ROLLBACK.